Earlier versions of this R01 grant focused on the role of neurotrophic signaling pathways in the neural plasticity induced in the ventral tegmental area (VTA) nucleus accumbens (NAc) reward circuit by chronic exposure to stimulant or opiate drugs of abuse, with a particular focus on the molecular mechanisms by which these drugs alter the morphology of VTA and NAc neurons. In this renewal application, we propose to focus on one major aspect of this effort: stimulant and opiate regulation of the morphology of NAc medium spiny neurons (MSNs). While there has been considerable work on this topic, major questions persist, given different laboratories' use of contingent vs. noncontingent drug exposure and of different methods to quantify dendritic morphology, and their focus on different times after drug withdrawal and on different subregions of NAc (core vs. shell). A further complicating factor is that few studies have considered different effects of drug exposure on the two main subtypes of NAc MSNs, those expressing predominantly the D1 vs. D2 dopamine receptor. The goal of this grant is to characterize, in a comprehensive cell type and subregion specific manner, the effect of cocaine or heroin self-administration, over the life cycle of self-administration behavior (acquisition, withdrawal, extinction, and relapse), on the number and morphology of NAc dendritic spines. We will also gain insight into input specificity of such dendritic plasticity as ell as the postsynaptic strength of spines under control and drug self-administration conditions by use of optogenetic and 2 photon imaging approaches. In parallel, we will continue our innovative studies that have provided novel insight into the molecular basis by which chronic drug self-administration induces this postsynaptic plasticity of NAc MSN dendritic spines. This work, in concert with ongoing studies in many other laboratories focused largely on complementary aspects of glutamatergic synaptic plasticity in drug abuse models, will help establish the ways in which such dendritic plasticity contributes to a state of addiction. As well, these studies will hep us capitalize on our improved understanding of drug-induced dendritic plasticity in NAc MSNs by identifying target genes suitable for future drug discovery efforts.